Interleukin 2 (IL-2) and soluble Interleukin 2 receptor (IL-2R) were assayed in serum and bronchoalveolar lavage fluid (BALF) from patients with farmer's lung. Patients with farmer's lung had significantly higher levels of soluble IL-2R in serum and BALF than normal controls. The level of soluble IL-2R was correlated with the disease activity, and fell to within the normal range after successful treatment with corticosteroids. On the other hand, asymptomatic dairy farmers with precipitating antibodies against Micropolyspora faeni antigen or Thermoactinomyces vulgaris antigen had the same levels of soluble IL-2R in their serum and BALF as normal controls. IL-2 levels in serum and BALF were significantly higher in patients with farmer's lung than in normal controls; however, asymptomatic patients had low levels comparable to those of normal controls. These results indicate that soluble IL-2R could be a sensitive parameter for evaluation of the activity of this disease. It is also suggested that the IL-2/IL-2R system is important in the pathogenesis of hypersensitivity pneumonitis.